Minoan eruption

The Minoan eruption of Thera, also referred to as the Thera eruption or Santorini eruption, was a major catastrophic volcanic eruption (Volcanic Explosivity Index (VEI) = 6 or 7, Dense-rock equivalent (DRE) = 60 km3)[1] which is estimated to have occurred in the mid second millennium BCE.[2] The eruption was one of the largest volcanic events on Earth in recorded history.[3][4][5] The eruption devastated the island of Thera (also called Santorini), including the Minoan settlement at Akrotiri -- as well as communities and agricultural areas on nearby islands and on the coast of Crete.

The eruption seems to have inspired certain Greek myths[6] and may have caused turmoil in Egypt.[7][8] Additionally, it has been speculated that the Minoan eruption and the destruction of the city at Akrotiri provided the basis for or otherwise inspired Plato's story of Atlantis.[9][10]

Contents

Eruption

Volcanic craters on Santorini today

Background

Geological evidence shows the Thera volcano erupted numerous times over several hundred thousand years before the Minoan eruption. In a repeating process, the volcano would violently erupt, then eventually collapse into a roughly circular seawater-filled caldera, with numerous small islands forming the circle. The caldera would slowly refill with magma, building a new volcano, which erupted and then collapsed in an ongoing cyclical process.[11] Another famous volcano known to repeat a similar process is Krakatoa in Indonesia.

Immediately prior to the Minoan eruption, the walls of the caldera formed a nearly continuous ring of islands with the only entrance lying between Thera and the tiny island of Aspronisi.[11] This cataclysmic eruption was centered on a small island just north of the existing island of Nea Kameni in the centre of the then-existing caldera. The northern part of the caldera was refilled by the volcanic ash and lava, then collapsed again.

Magnitude

Recent research by a team of international scientists in 2006 revealed that the Santorini event was much larger than the original estimate of 39 km3 (9.4 cu mi) of Dense-Rock Equivalent (DRE), or total volume of material erupted from the volcano, that was published in 1991.[1] With an estimated DRE in excess of 60 km3 (14 cu mi),[1][5] the volume of ejecta was approximately 100 km3 (24 cu mi), placing the Volcanic Explosivity Index of the Thera eruption at 6 or 7. This was up to four times what was thrown into the stratosphere by Krakatoa in 1883, a well-recorded event. The Thera volcanic events and subsequent ashfall probably sterilized the island, as occurred on Krakatoa. Only the Mount Tambora volcanic eruption of 1815, Lake Taupo's Hatepe eruption around 180, and perhaps the Baekdu Mountain eruption around 970 released more material into the atmosphere during historic times.[3][4]

Sequence

On Santorini, there is a 60 m (200 ft) thick layer of white tephra that overlies the soil clearly delineating the ground level prior to the eruption. This layer has three distinct bands that indicate the different phases of the eruption:[12] Studies have identified four major eruption phases, and one minor precursory tephra fall. The thinness of the first ash layer, along with the lack of noticeable erosion of that layer by winter rains before the next layer was deposited, indicate that the volcano gave the local population a few months' warning. Since no remains have been found at the Akrotiri site, this preliminary volcanic activity probably caused the island's population to flee. It is also suggested that several months before the eruption, Santorini experienced one or more earthquake(s), which damaged the local settlements.[13]

Intense magmatic activity of the first major phase (B01/Minoan A) of the eruption deposited up to 7 m. of pumice and ash, with a minor lithic component, southeast and east. Archaeological evidence indicated burial of man-made structures with limited damage. The second (B02/Minoan B) and third (B03/Minoan C) eruption phases involved pyroclastic flow activity and the possible generation of tsunamis. Man-made structures not buried during Minoan A were completely destroyed. The third phase was also characterized by the initiation of caldera collapse. The fourth, and last, major phase (B04/Minoan D) was marked by varied activity: lithic-rich base surge deposits, lahars, debris flows, and co-ignimbrite ash-fall deposits. This phase was characterized by the completion of caldera collapse, which produced tsunami.[14]

Geomorphological consequences

Mansions and hotels on the steep cliffs

Although the fracturing process is not yet known, the altitudinal statistical analysis indicates that the caldera had formed just before the eruption. During this period the area of the island was smaller and the southern and eastern coastlines appeared regressed. During the eruption period the landscape was covered by the pumice sediments. In some places, the coastline vanished under thick tuff depositions, and in others recent coastlines were extended towards the sea. After the eruption, the geomorphology of the island was characterized by an intense erosional phase, during which the pumice was progressively removed from the higher altitudes towards the lower ones.[15]

Volcanology

This Plinian eruption resulted in an estimated 30 to 35 km (19 to 22 mi) high plume which extended into the stratosphere. In addition, the magma underlying the volcano came into contact with the shallow marine embayment, resulting in a violent steam eruption.

The event also generated a 35 to 150 m (110 to 490 ft) high tsunami that devastated the north coast of Crete, 110 km (68 mi) away. The tsunami had an impact on coastal towns such as Amnisos, where building walls were knocked out of alignment. On the island of Anafi, 27 km (17 mi) to the east, ash layers 3 m (10 ft) deep have been found, as well as pumice layers on slopes 250 m (820 ft) above sea level.

Elsewhere in the Mediterranean there are pumice deposits which could have been caused by the Thera eruption. Ash layers in cores drilled from the seabed and from lakes in Turkey, however, show that the heaviest ashfall was towards the east and northeast of Santorini. The ash found on Crete is now known to have been from a precursory phase of the eruption, some weeks or months before the main eruptive phases, and would have had little impact on the island.[16] Santorini ash deposits were at one time claimed to have been found in the Nile delta,[17] but this is now known to be a misidentification.[18][19]

Date

The Minoan eruption is a key marker for the Bronze Age archaeology of the Eastern Mediterranean world. It provides a fixed point for aligning the entire chronology of the second millennium BCE in the Aegean, because evidence of the eruption is found throughout the region. Despite this evidence, the exact date of the eruption has been difficult to determine. For most of the twentieth century, archaeologists placed it at c. 1500 BCE, but, as radiocarbon dating techniques were applied to related archaeological material, this date appeared to be too young. Soon the dispute over the true date of the eruption grew heated and intense with the scientists divided in two camps: those who support the earlier seventeenth century BCE date, and those who support, on historical grounds, a date of c. 1500 BCE or slightly later.[20]

Archaeologists developed the Late Bronze Age chronologies of eastern Mediterranean cultures by analyzing the origin of artifacts (for example, items from Crete, mainland Greece, Cyprus or Canaan) found in each archaeological layer.[21] If an artifact's origin can be accurately dated, then it gives a reference date for the layer in which it is found. If the Thera eruption could be associated with a given layer of Cretan (or other) culture, chronologists could use the date of that layer to date the eruption itself. Since Thera's culture at the time of destruction was similar to the Late Minoan IA (LMIA) culture on Crete, LMIA is the baseline to establish chronology elsewhere. The eruption also aligns with Late Cycladic I (LCI) and Late Helladic I (LHI) cultures, but predates Peloponnesian LHI.[22] Archeological digs on Akrotiri have also yielded fragments of nine Syro-Palestinian Middle Bronze II (MBII) gypsum vessels.[21]

The Aegean prehistorians felt so confident about their calculations that they rejected early radiocarbon dates in the 1970s for LMI/LCI Thera, because radiocarbon suggested a date about a century earlier than the "traditional" dates.[23] At one time, data from Greenland ice cores seemed to support the radiocarbon dates. A large eruption identified in ice cores and dated to 1644 BCE (+/- 20 years) was suspected to be Santorini. However, volcanic ash retrieved from an ice core demonstrated that this was not from Santorini, leading to the conclusion that the eruption may have occurred on another date.[16] The late Holocene eruption of the Mount Aniakchak, a volcano in Alaska, is proposed as the most likely source of the minute shards of volcanic glass in the Greenland ice core.[24] Another method used to establish the date of eruption is tree-ring dating. Tree-ring data has shown that a large event interfering with normal tree growth in North America occurred during 1629-1628 BCE.[25] Evidence of a climatic event around 1628 BCE has been found in studies of growth depression of European oaks in Ireland and in Sweden.[26]
Bristlecone pine frost rings also indicate a date of 1627.
[27][28]
Procedural changes in how ice cores are interpreted would bring that
data more in line with the dendrochronological numbers.
[29]

Current radiocarbon estimates indicate that the eruption occurred between 1627 BCE and 1600 BCE.[30] Samples of wood, bone, and seed collected from various locations in the Aegean, including Santorini, Crete, Rhodes and Turkey, were analyzed at three separate labs in Oxford, Vienna, Austria, and Heidelberg, Germany, in order to minimise the chance of a radiocarbon dating error. Results of the analysis indicated a broad dating for the Thera event between 1660 to 1613 BCE.[31][32] Also that year the radiocarbon-indicated date of the eruption of Thera was narrowed to between 1627 and 1600 BCE, with a 95% probability of accuracy, after researchers analyzed material from an olive tree that was found buried beneath a lava flow from the volcano.[33] If radiocarbon dating is accurate, there would be significant chronological realignment of several Eastern Mediterranean cultures.[34][35]

Although radiocarbon consistently indicates a 1600 BCE eruption dating, some archeologists still believe that the date is contradicted by findings in Egyptian and Theran excavations. For example, buried Egyptian and Cypriot pottery found on Thera were dated to a later period than the radiometric dates for the eruption, and, since the conventional Egyptian chronology has been established by numerous archaeological studies, the exact date of the eruption remains controversial.[30][36][37]

Climatic effects

Hydrogeologist Philip LaMoreaux asserted in 1995 that the eruption caused significant climatic changes in the eastern Mediterranean region, Aegean Sea and much of the Northern Hemisphere,[38] but this was forcefully rebutted by volcanologist David Pyle a year later.[39]

Around the time of the radiocarbon-indicated date of the eruption, there is evidence for a significant climatic event in the Northern Hemisphere. The evidence includes failure of crops in China (see below), as well as evidence from tree rings, cited above: bristlecone pines of California; bog oaks of Ireland, England, and Germany; and other trees in Sweden. The tree rings precisely date the event to 1628 BCE.[25][26]

Historical impact

Minoan civilization

Excavation of Akrotiri on Thera

The only gold object found at the excavation of Akrotiri, a small sculpture of an ibex that was hidden under a floor; a thorough evacuation in advance of the catastrophe must have occurred since few artifacts and no corpses were buried in the ash

The eruption devastated the nearby Minoan settlement at Akrotiri on Santorini, which was entombed in a layer of pumice.[40] It is believed that the eruption also severely affected the Minoan population on Crete, although the extent of the impact is debated. Early theories proposed that ashfall from Thera on the eastern half of Crete choked off plant life, causing starvation of the local population.[41] However, after more thorough field examinations, this theory has lost credibility, as it has been determined that no more than 5 mm (0.20 in) of ash fell anywhere on Crete.[42] Other theories have been proposed based on archeological evidence found on Crete indicating that a tsunami, likely associated with the eruption, impacted the coastal areas of Crete and may have severely devastated the Minoan coastal settlements.[43][44][45] A more recent theory is that much of the damage done to Minoan sites resulted from a large earthquake that preceded the Thera Eruption.[46]

Significant Minoan remains have been found above the Late Minoan I era Thera ash layer, implying that the Thera eruption did not cause the immediate downfall of the Minoans. As the Minoans were a sea power and depended on their naval and merchant ships for their livelihood, the Thera eruption likely caused significant economic hardship to the Minoans, and the loss of empire in the long run.

Whether these effects were enough to trigger the downfall of the Minoan civilization is under intense debate. The Mycenaean conquest of the Minoans occurred in Late Minoan II period, not many years after the eruption, and many archaeologists speculate that the eruption induced a crisis in Minoan civilization, which allowed the Mycenaeans to conquer them easily.[47]

Chinese records

A volcanic winter from an eruption in the late 17th century BCE has been claimed by some researchers to correlate with entries in Chinese records documenting the collapse of the Xia dynasty in China. According to the Bamboo Annals, the collapse of the dynasty and the rise of the Shang dynasty, approximately dated to 1618 BCE, were accompanied by "'yellow fog, a dim sun, then three suns, frost in July, famine, and the withering of all five cereals".[7]

Heavy rainstorms which devastated much of Egypt, and were described on the Tempest Stele of Ahmose I, have been attributed to short-term climatic changes caused by the Theran eruption.[7][8][49]

While it has been argued that the damage from this storm may have been caused by an earthquake following the Thera Eruption, it has also been suggested that it was caused during a war with the Hyksos, and the storm reference is merely a metaphor for chaos, upon which the Pharaoh was attempting to impose order.[50]

There is a consensus that Egypt, being far away from areas of significant seismic activity, would not be significantly affected by an earthquake in the Aegean. Furthermore, other documents, such as Hatshepsut's Speos Artemidos, depict similar storms, but are clearly speaking figuratively, not literally. Research indicates that this particular stele is just another reference to the Pharaoh's overcoming the powers of chaos and darkness.[50]

In The Parting of the Sea: How Volcanoes, Earthquakes, and Plagues Shaped the Exodus Story,[51] geologist Barbara J. Sivertsen seeks to establish a link between the eruption of Santorini ca. 1628 BCE and the Exodus from Egypt in the Bible.

Greek traditions

The eruption of Thera and volcanic fallout may well have inspired the myths of the Titanomachy in Hesiod's Theogony.[6] The background of the Titanomachy may derive from the Kumarbi cycle, a Bronze Age Hurrian epic from the Lake Van region.

However, the Titanomachy itself could have picked up elements of western Anatolian folk memory as the tale spread westward. Hesiod's lines have been compared with volcanic activity, citing Zeus' thunderbolts as volcanic lightning, the boiling earth and sea as a breach of the magma chamber, immense flame and heat as evidence of phreatic explosions, among many other descriptions.[52]